As disclosed in “CMOS-Compatible AlN Piezoelectric Micromachined Ultrasonic Transducers,” Stefon Shelton et al., 2009 IEEE International Ultrasonics Symposium Proceedings, pp. 402-405 (hereinafter “non-patent document 1”), a piezoelectric device including a substrate and a piezoelectric layer and an electrode which are provided above the substrate has conventionally been known. Such a piezoelectric device is used for a filter, an actuator, a sensor, and a pMUT (piezoelectric Micromachined Ultrasonic Transducers).
U.S. Patent Publication No. 2010/0195851A1 (hereinafter “patent document 1”) discloses in paragraphs 0017 to 0023 and FIGS. 1A and 1B, a MEMS device including a transducer, a substrate, and a temperature compensating element as one example of such a piezoelectric device. The transducer is, for example, a piezoelectric ultrasonic transducer which vibrates in a flexural mode and is arranged above the substrate. The temperature compensating element is, for example, a heater element, and it is arranged on the substrate around the transducer on a lower surface side thereof.
U.S. Patent Publication No. 2010/0134209A1 (hereinafter “patent document 2”) discloses in paragraphs 0070 to 0076 and FIGS. 2A and 2B, a sensor array including a sensor element, a semiconductor substrate, and a heater element. The sensor element is a piezoelectric resonance element having a piezoelectric layer and two electrodes and arranged on the substrate. The heater element is arranged around the sensor element on the substrate.
The heater element described in these documents is used for adjusting a temperature of a piezoelectric device for stabilizing an oscillation frequency and removing moisture which is attached to a surface of the piezoelectric device.
Such a piezoelectric device is disadvantageous in great variation in resonance frequency (hereinafter also referred to as “frequency variation”) caused during manufacturing. There are some factors determining frequency variation such as a dimension including a thickness and a length of the device. In particular, the most important factor is a stress in a member forming a membrane portion. A member forming the membrane portion includes a piezoelectric layer and an electrode.
A stress in the member forming the membrane portion may vary depending on a manufacturing method such as a sputtering method or a sol-gel method. A piezoelectric layer composed of AlN is often formed with the sputtering method. When the piezoelectric layer composed of AlN is formed with the sputtering method, a stress in the piezoelectric layer may greatly vary in a wafer plane due to a temperature distribution in a substrate or a plasma distribution. Consequently, frequency variation of the piezoelectric device is great.
Japanese Patent Laying-Open No. 2001-85752 (hereinafter “patent document 3”) discloses heat treatment in a heat treatment furnace for mitigating a stress in a functional film in a semiconductor sensor. In the heat treatment in the heat treatment furnace, however, the entire piezoelectric device is heated with a temperature of the heat treatment furnace being increased to a desired temperature, and hence the heat treatment is disadvantageous in time and cost.
Japanese Patent Laying-Open No. 10-256570 (hereinafter “patent document 4”) discloses formation of a thin film formed of a piezoelectric material while a substrate is heated with a local heating portion as a heater element formed on the substrate in order to form a thin film excellent in characteristics in a thin-film device. A temperature distribution in a substrate or a plasma distribution in the sputtering method, however, affects magnitude of a stress in the piezoelectric layer composed of AlN, and hence great frequency variation caused during manufacturing is not necessarily overcome even by this method.